Conventional drainage systems of the type to which the present invention relates typically comprise a horizontally extending perforated conduit disposed within a drainage trench and surrounded by a quantity of loose aggregate material, such as rock or crushed stone, and covered with compacted soil. The space between the conduit and the ground occupied by the aggregate serves to define a drainage cavity in fluid communication with the perforations of the conduit.
An example of such a drainage system is found in the nitrification field of conventional ground absorption sewage treatment and disposal systems wherein effluent discharges from a septic tank through the perforated vent pipe of a nitrification line which is surrounded by a specified minimum volume of aggregate material, such as rocks or crushed stone. The purpose of the nitrification field is to create a storage area for sewage effluent until it can be absorbed by the soil (percolate). The aggregate material acts to maintain the boundaries of the storage area, prevent blockage of the pipe perforations, promotes the beneficial effects of biomat development (aerobic bacteria organisms that act on the sewage colloidal materials to reduce them to soil) and retards development of the clogging mat (i.e. the mechanical loss of infiltrative capacity at the soil surface interface due to suspended solids, bacteria growth and ferrous sulfide precipitation).
Conventional ground absorption disposal systems of that type utilize septic tanks and nitrification lines of adequate construction and design volume in accordance with provisions of local building and health codes governing the sanitary disposal of wastes. The effluent from the septic tank flows by gravity to an approved nitrification line where the soil provides for final treatment and disposal of the sewage. The actual processing depends upon the class of soil texture (whether sand, loam, clay or mixtures and variations of the same) into which the nitrification line extends. The square footage of area needed for the nitrification field in a trenched system depends on the rate and volume of effluent to be disposed. The perforated conduit serves the dual purpose of delivering the effluent to the aggregate filled cavity for absorption into the soil and to vent sewage gases to prevent local concentration thereof.
The installation of conventional nitrification lines involves digging a trench and depositing loose aggregate in the form of rock or crushed stone materials into the trench for a minimum depth. The horizontal conduit in the form of a perforated pipe is then laid down on the base aggregate and surrounded by additional quantities of the aggregate to give required minimum vertical and horizontal dimensions of aggregate surrounding the pipe. The trench, pipe and aggregate volume and thickness dimensions must all conform to local sanitary disposal codes and specifications. A typical system might, for example, utilize a four inch minimum inside diameter Schedule 40 PVC pipe or equivalent, set in substantially horizontal orientation with a minimum fall or not less than 1/8 inches per foot. The tubing may, for example, be four- or six-inch diameter corrugated plastic tubing complying with applicable ASTM standards having three rows of holes each 1/2 to 3/4 inches in diameter, spaced longitudinally on approximately four-inch centers. The rows of holes may be equally spaced 120.degree. on centers about the periphery of three rows may be located in the lower portion of the tubing with the outside rows being approximately 120.degree. on centers.
The nitrification trenches are constructed as level as possible, with the fall in a single trench bottom meeting local maximum requirement, such as not to exceed 1/4 inches in ten feet. Trench diameters are chosen to prevent too rapid a rate of sewage discharge or too great a strength into the zone of aeration where organic effluent conversion into soil occurs. Typical trench diameters are about three feet in width and two to three feet in depth.
The loose aggregate placed in the trench to surround the vent pipe is typically required to be clean washed gravel (rock or crushed stone) which is graded or sized between 3/4 and 2-1/2 inches. The gravel in a typical system is required to be placed a minimum of one foot deep with at least six inches below the pipe and two inches over the pipe, and to be distributed uniformally across the trench bottom and over the pipe. Soil cover over the nitrification field is specified to be a depth of at least six inches, or so, with the finished grade over the field being landscaped to prevent ponding of surface water and to encourage surface water runoff to be diverted away from the nitrification field.
Other mechanisms such as effluent distribution devices (including distribution boxes, flow dividers and flow diversion devices), grease traps, or the like are also included as required and approved by state and/or local health regulations.
Stepdowns or dropboxes are used where topography prohibits the placement of nitrification trenches on level grade. Their placement and design is in accordance with local specifications.
Conventional installation of drainage fields such as those described, requires the installation of rock, shell, or other labor intensive material under, around and over corrugated tubing. Such installation methods do not lend themselves well to installations involving adverse terrain, working area or unskilled worker abilities. The requirement for uniformity and inspections for compliance with state and local codes makes the installation process tedious and time consuming.